It is standard practice to compress a fluent mass of explosive powder into a rigid body suitable for ordinance and related purposes. Obviously this procedure must avoid anything that could set off the explosive.
The normal method is to charge an appropriate metered quantity of the powder from which the body is to be made into a cup-shaped mold or die, and to compress it therein with a complementarily shaped stamp or punch. When the powder being used is quite fine and fluent as is common in the production of explosives, it is necessary that the punch fit very snugly into the die. This creates the problem of trapping and compressing air in the die with the powder. If the air is not allowed to escape, its compression will heat it, possibly setting off an explosion. It is also possible for the resultant body to be made somewhat porous and friable by gas inclusions in it.
Accordingly it has been suggested to provide radially throughgoing holes in the wall of the mold or die slightly above the level where the upper surface of the charged-in mound of material should be. Thus as the punch descends in the die it will expel the air through these holes, the pressing chamber formed between the punch and die only being fully closed off when the punch passes these openings, which should happen just before it comes into contact with the powder being pressed.
Such an arrangement cannot readily be used when the die is held, as is common, in a liquid bath to maintain it at a predetermined temperature due to the danger of leakage into the die. Furthermore if the material is not smoothly mounded in the die it can rise at one location up over one of the holes so that this portion of the powder is blown out this hole as the die descends, possibly blocking this hole. Finally such a system requires that a given mold or die always be used with a given quantity of powder since the holes cannot be moved, and even when it works well a small amount of air is compressed into the body due to the short distance the punch must travel for safety's sake between the position when it covers the holes and when it contacts the mass being compressed.
Hence it has been suggested to operate in a vacuum. Such a procedure entails considerable equipment costs. The punches and dies must be contained in a vacuum chamber and some means must be provided for feeding the explosive powder to the dies in the chamber. Overall the equipment needed for operation in this manner is prohibitively complex and expensive.